In order to be able to regulate, as desired, the frequency of oscillation of an oscillator circuit of the PIERCE type, i.e. a crystal oscillator having two capacitors, each of which is connected between the crystal resonator and the common or ground terminal of the circuit, it has already been proposed that these capacitors normally incorporated in the integrated circuit should be replaced by controllable-capacitance capacitors in the form of discrete components which are connected to suitable terminals of the integrated circuit.
Accompanying FIG. 1 shows an application of that known design to an oscillator of the type described in U.S. Pat. No. 4,013,979.
The circuit shown in FIG. 1 comprises an integrated circuit 1 of CMOS type, a quartz resonator 2, an electrical power supply source 3 and two circuits 4a and 4b which are formed from discrete components and which comprise capacitors 5a and 5b, respectively, of the type having a capacitance which varies continuously in dependence on the voltage applied thereto.
The integrated circuit 1 comprises a pair of terminals 6 and 7 which are connected to the power supply source 3, and a second and a third pair of terminals as shown at 8a, 8b and 9a, 9b for connection to the respective circuits 4a and 4b.
The integrated circuit 1 comprises a MOS transistor 10, for example a n-channel transistor, which is polarized by a resistor 11 connected between the gate 10a and the drain 10b of the transistor 10, the source of which is indicated by reference numeral 10c. The current flowing in the controlled current path, or drain-source path, of the transistor 10 is determined by a current source 12 for example a p-channel MOS transistor, which is connected in series with said drain-source path between the supply terminals 6 and 7 of the integrated circuit 1. The source 12 provides a current controllable in dependence on a control signal applied to its control input 12a which is for example the gate of said p-channel transistor. This control signal is supplied by a regulator circuit 13 responsive to the oscillation signal present at the gate 10a of the transistor 10. The circuit 13 is designed for supplying a control signal which is a suitable function of the amplitude of said oscillation signal, whereby the circuit 13 and the current source 12 form a regulating loop circuit for limiting the amplitude of the oscillating signal to a low value. U.S. Pat. No. 4,013,979 shows several embodiments for the regulator circuit 13.
One plate of each capacitor 5a and 5b is connected to the terminals 8b and 9b, respectively. The other plates 50a, 50b of the capacitors 5a, 5b, respectively, are connected, via a decoupling capacitor 16a and 16b, respectively, to the terminals 8a and 9a, respectively, of the integrated circuit 1. The terminals 8b and 9b are connected to the negative one 7 of the supply terminals, which is the common- or ground-terminal of the circuit. Matching resistors 15a and 15b are interposed between control terminals 14a and 14b, respectively, and the plates 50a and 50b, respectively, of the capacitors 5a and 5b, respectively. Each plate 50a and 50b is also connected to a respective terminal 2a and 2b, respectively, for connection to the quartz 2. Finally, the decoupling capacitors 16a and 16b are interposed between the terminals 8a and 9a, respectively, and the common points A and B, respectively between the resistors 15a and 15b, respectively, and the capacitors 5a and 5b, respectively.
Thus, it is possible to adjust the capacitance of each of the capacitors 5a and 5b by applying a suitable control voltage Ua and Ub between the common terminal 7 and respective control terminals 14a and 14b of the respective circuits 4a and 4b.
The applicant has found that the circuit shown in FIG. 1 does not generally prove satisfactory by virtue of the narrowness of the range of frequency values at which the oscillator can be set, and also by virtue of a relatively high level of current consumption. The first disadvantage referred to above is primarily due to the parasitic capacitances represented in FIG. 1 by references C1a, C2a, C3aand C1b, C2b and C3b which capacitances are liable to short-circuit the respective capacitors 5a and 5b, in the high-frequency mode. It has been found that essentially parasitic capacitances C1a and C2a and C1b and C2b are of relatively high values; this is because the circuits 4a and 4b comprise discrete components.
Further, German Pat. No. 2.638.055 discloses a crystal oscillator provided with means for adjusting the frequency of the oscillations of the crystal. This adjusting means comprises a set of capacitors of the type known in the literature as "Bicap". It is known that such a capacitor may have one or the other of two different values depending on whether or not the voltage applied to it exceeds a threshold value.
Thus the frequency of this known oscillator is adjusted in a discontinuous manner. Furthermore, the circuit according to this prior art is rather complicated and expensive because it needs a large number of Bicaps in order to be able to set the frequency over a wide range of values.
Accordingly one object of the present invention is to propose a novel circuit for establishing and sustaining the oscillations of a resonator, which circuit permits adjustment of the frequency of oscillation of the resonator over a sufficiently wide range of values, for example with a range .DELTA.f/f of the order of .+-.200 ppm, at high frequencies, in particular of the order of from a few MHz to 20 MHz, for example 16 MHz which is a typically used telecommunication frequency.